The present invention relates to the pneumatic injection of a carbureted mixture into a combustion chamber of a two-stroke internal-combustion engine.
More precisely, the device according to the invention allows control of the rate of flow and of the pressure of the air under pressure used for spraying pneumatically the fuel into the combustion chamber.
In the particular case of a pneumatic injection two-stroke engine such as disclosed, for example, in patent FR-2,575,523, the air under pressure used for spraying fuel pneumatically may come from the pump crankcase and only a small proportion of this air is used for pneumatic injection, with the major part of this air taking part in the scavenging of the cylinder.
Regulation and control of the pressure and of the flow rate of the air under pressure necessary for the pneumatic injection of fuel are two very important parameters which may contribute towards improving the combustion very substantially.
According to the invention, the combustion quality will be notably be improved with respect to the stratification conditions of the gases in the combustion chamber, at low speeds and low loads.
The present invention allows adaptation of the geometry of the pneumatic fuel injection jet, as well as of its distribution in space and in time, in order to optimize its penetration and its mixing with the gases already contained in the combustion chamber of the engine.
It is known that, in a two-stroke engine such as illustrated, for example, in FIG. 1, the pneumatic injection system may comprise a pipe opening into the combustion chamber, intended for conveying a gas generally under pressure used for spraying the fuel. The fuel may be fed into the pipe through a metering system which opens preferably close to the end of the pipe opening into the combustion chamber. An intermittent sealing element such as, for example, a valve, generally located at the place in which the pipe opens, allows, as soon as it is open, the fuel to be sprayed into the combustion chamber by compressed gas used. Patents FR-2,575,521 and FR-2,575,522 show injectors of this type.
The pressure source used in these examples may come, as illustrated by French patent FR-2,575,523, totally or partly from the pump crankcase of the cylinder in which injection takes place or of another cylinder whose angular offset is used to modify the injection pressure.
Various solutions have been proposed to improve such injection systems, notably in order to increase the pressure of the compressed gas. Thus, French patent FR-B1-2,592,436 recommends placing a check valve on the pipe delivering gas under pressure so that part of the pipe constitutes a capacity for storing gas under pressure.
Although these various designs allow the injection pressure to be raised, they are not modulatable as a function of the various operating conditions of the engine because they are mainly controlled through pressure differences. Thus, if the pneumatic injection system is adjusted so as to obtain an optimum spraying with high loads and high speeds, poorer performances might be achieved at lower speeds. In fact, for example when the opening of the controlled system is optimized to obtain the required spraying quality and penetration at high loads and high speeds, if the same pressure level of compressed air is maintained for lower is speeds, the introduction of the compressed air-fuel mixture may be too fast, driving, for example, the fuel towards the exhaust port too rapidly.
This example is illustrated in FIG. 2 which shows pressure curves as a function of the crankshaft angle. Curve C shows the evolution of the pressure in the cylinder and curves A and B show respectively the compressed air pressure (in the supply pipe) at high speed and at low speed.
The injection time I substantially stretches out around the exhaust valve closing FE.
In FIG. 2, OE relates to the exhaust valve opening, PMH and PMB respectively relate to the top dead center and to the bottom dead center.
FIG. 2 shows a case where the compressed air pressure decreases too quickly at low engine speeds (curve B), and quickly reaches the cylinder pressure (curve C), long before the closing of the valve which corresponds to the end of the injection. The result of this is a too sudden inflow of carbureted mixture, which may be harmful at high loads and low speed because a significant amount of fuel may then reach the exhaust and escape in the atmosphere without burning. This phenomenon may also be harmful at low loads because the carbureted mixture injected too rapidly and too suddenly mixes too quickly with the residual waste gases contained in the cylinder and is no longer flammable at the time of the ignition or of the expected starting of the combustion.
On the other hand, as also shown in FIG. 2, if the carbureted mixture is introduced too rapidly, with too fast a compressed air pressure drop, a phenomenon known as a backflow, where the gases flow back from the cylinder to the level of the pneumatic injector, may occur, and the cylinder pressure becomes higher than the compressed air pressure upstream from the pneumatic injector. This backflow, which takes place in the zone II in FIG. 2, is not favorable because it decreases the cylinder compression and may feed residual gases into the pneumatic injector, which is not desirable to obtain good conditions of flammability of the carbureted mixture during the next cycle.